This invention relates to a control circuit for use in a switching-type regulator and, more particularly, to an improved control circuit which is used when the regulator first is turned ON so as to reset the regulating circuitry to an initial operating state and to achieve a gradual increase toward the operating voltage level in the output of the regulator.
One type of a switching regulator includes a source of unregulated DC voltage, such as may be derived by a full wave rectifier from AC supply mains, a switching circuit to periodically interrupt the unregulated DC voltage so as to derive an AC voltage therefrom, and an output rectifier which rectifies the AC voltage to produce a regulated DC voltage. The level of this regulated DC voltage is determined by the duty cycle of the switching circuit. That is, if the switching circuit is turned ON for only a small portion of each AC period, the integrating capacitor included in the output rectifier is charged to a corresponding small DC voltage level. As the duty cycle of the switching circuit increases, the DC voltage to which the integrating capacitor is charged correspondingly increases. Accordingly, in order to regulate this DC voltage to maintain a constant, desired level, the output voltage from the regulator is compared to a predetermined reference voltage, and any difference therebetween is used to control the duty cycle of the switching circuit in a manner which corrects, or compensates, for errors or changes in the regulator output voltage. As one example, the switching circuit is driven by a pulse width modulating generator which generates drive pulses whose durations are controlled as a function of the detected changes or variations in the regulator output voltage.
During an initial start-up operation, that is, when the regulator first is turned ON, such as by first connecting the regulator to AC supply mains or by first closing a power supply switch, the initial AC output voltage produced by the regulator is equal to zero. Consequently, a maximum error voltage may be supplied to the pulse width modulating pulse generator, thus resulting in drive pulses of maximum duration. Such drive pulses produce an abrupt, large increase in the DC output voltage produced by the regulator. Such an abrupt, large increase in the output voltage is not desirable. Furthermore, in those regulators which use a transformer to couple the AC voltage produced by the switching circuit to the ouput rectifier, such an abrupt, large increase in the output voltage, which is due to an abrupt increase in the duty cycle of the switching circuit, may cause a ringing in the core of the transformer.
In order to avoid this sudden start-up operation, a so-called "soft start" function is performed. This may be achieved by charging the capacitor with the aforementioned error voltage, that is, the voltage difference between the DC output voltage of the regulator and the desired reference level thereof. As this capacitor charges, its output voltage correspondingly increases, and this increasing voltage is used to effect an increase in the pulse duration of the drive pulses which are supplied to the switching circuit by the pulse width modulating pulse generator. Thus, rather than providing a sudden increase in the duty cycle of the switching circuit, the duty cycle thereof increases in a relatively gradual manner. Consequently, the DC output voltage of the regulator increases gradually. The time constant of this "soft start" operation may be on the order of 1 to 3 seconds.
The switching circuit of the aforedescribed type also may be provided with a protection circuit which prevents the regulator from producing a dangerously high over-current. Such a protection circuit may include a current detecting device which, when an over-current condition is detected, triggers a flip-flop circuit which, in turn, limits or disables the operation of the pulse width modulating pulse generator. If the pulse generator is disabled thereby, the DC output voltage of the regulator may be reduced to zero. On the other hand, if the triggered flip-flop circuit functions merely to limit the operation of the pulse generator, the duration of the pulses generated thereby may be limited to a relatively short portion of the AC period so as to reduce th AC output voltage of the regulator to a relatively low, safe level. In any event, in order to ensure proper initial operation of the regulator, it is necessary that this flip-flop circuit be reset when the regulator first is turned ON.
In order to rest the aforementioned flip-flop circuit for the purpose described above, when the regulator first is turned ON, a capacitor may be charged rapidly to a voltage level which is sufficient to carry out this reset operation. The time constant for charging such a capacitor may be on the order of 1 to 10 msec.
In order to carry out the soft start and reset operations discussed above, it has been thought heretofore that two separate timing circuits, including two separate capacitors, must be provided. In view of the marked difference between the soft start time constant and the reset time constant, it had been thought that successful operation with a single timing capacitor is not feasible.
Furthermore, and as may be appreciated, after the flip-flop circuit is reset and after the soft start operation is carried out, the timing circuits, and particularly the capacitors therein, which are used for these operations are no longer needed. That is, during normal operation of the regulator, these timing circuits and capacitors perform no useful purpose. Therefore, if the regulator is formed as an integrated circuit, it is preferred that these capacitors not be included therein. This means that two pairs of external terminals, one pair for each capacitor, must be provided in order to connect such capacitors to the regulator. The cost of fabricating an integrated circuit is influenced substantially by the number of external terminals which must be provided therewith. Hence, by requiring two pairs of external terminals, a regulator of integrated circuit construction is subject to relatively high manufacturing costs.
While the use of a single capacitor would contribute favorably to lower manufacturing costs of such an integrated circuit regulator, as mentioned above, it had not been thought feasible to eliminate the reset or soft start capacitor because of the substantial difference in the time constants attending the reset and soft start circuits. Furthermore, it is not advisable to seek a compromise in the reset and soft start time constants. If the time constant of the reset circuit is increased, it is possible that a proper reset pulse might not be produced. Hence, the flip-flop circuit might not be reset, and proper operation of the regulator is inhibited. Conversely, if the time constant of the soft start circuit is reduced, the initial DC output voltage of the regulator may rise too rapidly, thus defeating the purpose of the soft start operation. Thus, a compromise time constant for use in both the reset and soft start circuits is not practical.